1. Field of the Invention
The present invention relates to brake mechanisms for applying sliding friction to retard movement of spring balanced window sashes running in vertical sash runs. In particular, the invention relates to such mechanisms which utilize the tension of the spring acting upwardly and the weight of the window acting downwardly to produce a frictional force to assist in holding the window sash in a selected position in a vertical sash run.
2. The Prior Art
A basic problem which has been encountered in connection with spring balanced window sash mechanisms in the past is that a frictional force is sometimes needed to hold the sash in a desired position in a vertical sash run. When the sash is pushed downwardly, gravity assists the operation so that frictional forces may easily be overcome. However, particularly when it is desired to raise the sash, the operation is facilitated if such frictional forces may be diminished. Theoretically, a properly selected balance spring may exactly balance the weight of the sash at a particular, singular vertical position; however, such positioning seldom is encountered in actual practice. Accordingly, a friction brake capable of applying sufficient friction to hold the sash in all vertical positions and yet capable also of permitting movement of the sash during opening and/or closing operations has been sought. From a practical viewpoint, there has been a need for such a brake device which is capable of applying the required frictional force generally uniformly so as to prevent excessive forces and the resultant jamming and distortion of the braking and guidance mechanism.
A previous device which has found some use in the field is described in U.S. Pat. No. 4,571,887. The device disclosed in this prior patent incorporates interferring wedges which travel together in a guide channel and which move relatively to apply friction by forcing the walls of the channel apart. However, such devices often malfunction because the friction between the wedging surfaces of the relatively movable elements of the interferring wedge of the '887 device inherently inhibits relative movement of the wedging elements in a direction to effect release of the brake, particularly when upward movement of the sash is desired.
A previous improvement directed to avoiding the problems of the prior art and particularly the problems encountered in connection with the '887 device is disclosed in co-pending application Ser. No. 881,927, filed Jul. 3, 1986 and assigned to the assignee of the present application. The device of the '927 application solves many of the problems encountered in prior art devices; however, this device consisted of a number of intricate parts and is complicated in its manufacture and installation. Unlike the device of the '887 patent, which operates on the principle of a drum brake, the device of the '927 application operates on the principle of a disc brake.
Each of the prior art devices described above have utilized a pair of relatively vertically movable elements to create laterally directed frictional forces which were utilized to retard movement of the sash and assist in holding the same in a selected position in the vertical sash run.